17 June 2024
Looking to understand the function of the ignition coil and how to test it for faults? Check out this informational post for all the details you need.

Have you ever wondered how your car’s engine starts and ignites fuel to get you moving? Well, the ignition coil plays a crucial role in this process. Serving as a power-packed transformer, it converts the low voltage from your car’s battery into the high voltage needed to create sparks in your engine’s spark plugs. In my article, I will explore the function of the ignition coil in more detail and provide you with simple steps to test it for any potential faults. So, let’s dive in and uncover the secrets behind this essential component of your vehicle’s ignition system.

Function of the Ignition Coil

The ignition coil is a crucial component in a vehicle’s ignition system. Its primary function is to generate high voltage and convert low voltage from the battery to the spark plugs. Additionally, it plays a vital role in timing control and energy storage.

Generating High Voltage

One of the main functions of the ignition coil is to generate high voltage. This high voltage is necessary to create a spark at the spark plugs, which ignites the air-fuel mixture in the combustion chamber. The ignition coil steps up the low voltage from the battery to a high voltage, typically in the range of 20,000 to 50,000 volts, to enable the spark plugs to produce a strong spark.

Converting Low Voltage

Another important function of the ignition coil is to convert the low voltage from the battery into a higher voltage suitable for the spark plugs. The battery typically provides a voltage of 12 volts, which is not sufficient to create a spark in the combustion chamber. The ignition coil uses the principle of electromagnetic induction to step up the voltage and produce the high voltage required for ignition.

Timing Control

The ignition coil also plays a role in timing control. It is responsible for delivering the high voltage pulse to the spark plugs at the appropriate time in the engine’s combustion cycle. The timing is crucial for efficient engine operation and power delivery. The ignition coil receives signals from the engine control unit (ECU), which determines the timing based on various factors such as engine speed and load.

Energy Storage

In addition to generating high voltage and converting low voltage, the ignition coil also functions as an energy storage device. It stores electrical energy when the primary circuit is energized and releases it in the form of a high voltage pulse when the circuit is interrupted. This ensures that a strong and consistent spark is produced at the spark plugs, providing reliable combustion in the engine.

Components of an Ignition Coil

Understanding the components of an ignition coil is essential when it comes to testing and troubleshooting any issues. The main components of an ignition coil include the primary coil, secondary coil, core, terminal, insulation, and wiring.

Primary Coil

The primary coil is responsible for receiving the low voltage from the battery. It consists of a few hundred turns of thick wire wound around a core. The primary coil is connected to a power source, typically the ignition switch or the engine’s control module.

Secondary Coil

The secondary coil is a tightly wound coil of fine wire that is connected to the spark plugs. It is located inside the primary coil. When the primary coil is energized by the low voltage, it induces a high voltage in the secondary coil through electromagnetic induction. This high voltage is then delivered to the spark plugs.


The core is a magnetic component of the ignition coil, typically made of iron or a magnetic alloy. It helps to concentrate the magnetic field generated by the primary coil, enhancing the efficiency of the coil’s operation. The core also provides structural support to the coil assembly.


The terminal is the connection point between the ignition coil and the rest of the ignition system. It allows for the electrical connection to be established, ensuring that the coil receives the necessary voltage and delivers the high voltage output to the spark plugs.


Insulation is a critical component of the ignition coil as it prevents electrical leakage and ensures proper insulation between the primary and secondary coils. It is typically made of a high-quality insulating material, such as epoxy or plastic, that can withstand high temperatures and electrical stress.


The wiring in an ignition coil consists of the various electrical connections that allow for the flow of electrical current between the coil and the battery, as well as the spark plugs. Proper wiring is essential to ensure the ignition coil functions correctly and reliably.

How Does an Ignition Coil Work?

To comprehend how an ignition coil works, it is essential to understand the primary and secondary coil circuits, as well as the difference between a distributor system and a DIS (distributorless ignition system).

Primary Coil Circuit

The primary coil circuit is responsible for providing the low voltage required to induce a high voltage in the secondary coil. When the ignition switch is turned on, it energizes the primary coil circuit. This causes an electric current to flow through the primary coil, generating a magnetic field.

Secondary Coil Circuit

The secondary coil circuit is where the high voltage is produced. As the primary coil is energized and generates a magnetic field, it induces a high voltage in the tightly wound secondary coil. The high voltage is then delivered to the spark plugs through the distributor or directly in a DIS system.

Distributor System vs. DIS System

In a distributor system, the high voltage generated by the ignition coil is distributed to the respective spark plugs through a rotating distributor and rotor. The distributor ensures that each spark plug fires at the correct time in the engine’s combustion cycle.

In a DIS system, also known as a distributorless ignition system, there is no distributor. Instead, each spark plug has its own ignition coil, which is directly connected to the engine control module. This allows for precise control over the timing and intensity of the spark.

Symptoms of a Faulty Ignition Coil

Identifying the symptoms of a faulty ignition coil is essential for timely diagnostics and maintenance. Some common symptoms of a faulty ignition coil include engine misfires, poor fuel economy, stalling or rough idle, difficulty starting the engine, and backfiring.

Engine Misfires

One of the most common signs of a faulty ignition coil is engine misfires. If the ignition coil is failing, it may not generate enough voltage to create a strong spark at the spark plugs. This can result in incomplete combustion and lead to misfires, causing the engine to run rough or hesitate during acceleration.

Poor Fuel Economy

A faulty ignition coil can adversely affect fuel economy. When the ignition coil is not functioning correctly, it can result in incomplete combustion, leading to wasted fuel. This can cause a significant decrease in fuel efficiency and an increase in fuel consumption.

Stalling or Rough Idle

Another symptom of a faulty ignition coil is stalling or a rough idle. If the ignition coil is not providing enough voltage to the spark plugs, the combustion process may be disrupted, resulting in an unstable idle or even stalling of the engine.

Difficulty Starting the Engine

A failing ignition coil can also make it difficult to start the engine. If the coil is not generating sufficient voltage to produce a strong spark, the engine may struggle to ignite the air-fuel mixture. This can lead to extended cranking times or the engine failing to start altogether.


Backfiring, or the sudden explosion of fuel in the intake or exhaust system, can be a sign of a faulty ignition coil. When the ignition coil is not functioning properly, it can cause the air-fuel mixture to ignite at the wrong time, resulting in backfiring.

Tools Required for Testing

Testing an ignition coil for faults requires the use of specific tools to accurately diagnose any issues. The two primary tools needed for testing are a digital multimeter and an ignition coil tester.

Digital Multimeter

A digital multimeter is an essential tool for testing the resistance and voltage output of the ignition coil. It allows you to measure the resistance of the primary and secondary coil circuits, as well as the voltage output of the coil. A multimeter with the ability to measure both resistance and voltage is necessary for accurate testing.

Ignition Coil Tester

An ignition coil tester is a specialized tool designed specifically for testing ignition coils. It provides a quick and convenient way to check the primary and secondary coil resistance, as well as the voltage output of the coil. An ignition coil tester typically has multiple settings and connectors to match different ignition coil types.

Testing the Ignition Coil

Before testing the ignition coil, it is important to take some safety precautions to prevent any accidents or damage. Once the necessary precautions are in place, you can proceed with testing the ignition coil using the digital multimeter and ignition coil tester.

Safety Precautions

When testing the ignition coil, it is crucial to disconnect the battery to avoid any electrical shocks or damage. Additionally, make sure to follow the manufacturer’s guidelines and safety precautions for handling ignition system components. Always wear appropriate personal protective equipment, such as gloves and protective eyewear, to minimize the risk of injury.

Primary Coil Resistance Test

To test the resistance of the primary coil, set the digital multimeter to the resistance or ohms mode. Disconnect the electrical connectors from the ignition coil and place the multimeter leads on the terminal corresponding to the primary coil. Note the resistance reading displayed on the multimeter. Compare the reading to the manufacturer’s specifications to determine if the primary coil is within the acceptable range.

Secondary Coil Resistance Test

Similar to the primary coil resistance test, the secondary coil resistance can be measured using the digital multimeter. Set the multimeter to the resistance mode and connect the leads to the terminals of the secondary coil. Record the resistance reading and compare it to the manufacturer’s specifications.

Coil Output Test

To measure the voltage output of the ignition coil, use the ignition coil tester. Connect the tester to the ignition coil following the manufacturer’s instructions. Start the engine and observe the voltage reading on the tester. Compare the reading to the specified range to determine if the coil is producing the correct voltage output.

Interpreting Test Results

Once you have conducted the tests on the ignition coil, it is important to interpret the results correctly to determine if the coil is faulty or functioning properly. Different resistance and voltage values indicate various conditions of the ignition coil.

Primary Coil Resistance Values

The primary coil resistance values can vary depending on the specific ignition coil and the manufacturer’s specifications. Typically, the primary coil resistance should fall within a specified range, such as 0.4 to 1.2 ohms. If the resistance reading is significantly higher or lower than the specified range, it may indicate a fault in the primary coil.

Secondary Coil Resistance Values

Similar to the primary coil resistance, the secondary coil resistance values can vary depending on the type of ignition coil. It is essential to consult the manufacturer’s specifications for the acceptable range of resistance. A significant deviation from the specified range may indicate a problem with the secondary coil.

Coil Output Values

The voltage output of the ignition coil can also vary depending on the specific coil and the application. The acceptable range of coil output voltage is typically specified by the manufacturer. If the voltage output is significantly below or above the specified range, it may indicate a fault in the ignition coil.

Common Issues and Troubleshooting

While testing the ignition coil can help diagnose faults, it is also important to be aware of common issues that may arise and troubleshoot them accordingly. Some common issues include ignition coil overheating, electrical shorts, coil wire damage, and complete ignition coil failure.

Ignition Coil Overheating

Ignition coil overheating can occur due to various reasons, such as excessive current flow, poor coil grounding, or a faulty wiring connection. If the ignition coil gets too hot, it can lead to a decrease in performance and potentially cause damage. Ensure proper grounding and check for any loose or damaged wiring to prevent overheating.

Electrical Shorts

Electrical shorts can disrupt the flow of current and cause the ignition coil to malfunction. Shorts can occur due to damaged insulation, loose connections, or faulty wiring. It is important to inspect the ignition coil and wiring for any signs of damage or loose connections. Replace any damaged components and ensure a secure electrical connection.

Coil Wire Damage

Coil wire damage can result in poor electrical conductivity and affect the performance of the ignition coil. Inspect the ignition coil wire for any signs of wear, cracks, or breaks. If any damage is found, replace the coil wire to restore proper functionality.

Ignition Coil Failure

In some cases, the ignition coil may fail completely and stop functioning. This can occur due to various reasons, such as prolonged use, overheating, or manufacturing defects. If the ignition coil fails, it will need to be replaced with a new one to restore the ignition system’s proper operation.

Replacing a Faulty Ignition Coil

If testing confirms that the ignition coil is indeed faulty, it is necessary to replace it to ensure proper engine performance. The following steps outline the process of identifying the faulty coil, removing it, and installing a new one.

Identifying the Faulty Coil

Firstly, determine which ignition coil is faulty. Depending on the vehicle’s configuration, there may be multiple ignition coils. Refer to the service manual or consult a professional for guidance. Once identified, mark the faulty ignition coil for ease of replacement.

Removing the Faulty Coil

Disconnect the electrical connectors and wiring harness from the faulty ignition coil. Carefully remove any bolts or screws securing the coil in place. Gently pull the coil out of its mounting bracket, taking care not to damage any surrounding components.

Installing the New Coil

Before installing the new ignition coil, ensure it matches the specifications and requirements of your vehicle. Insert the new coil into the mounting bracket, ensuring a secure fit. Attach any bolts or screws to secure the coil in place. Reconnect the electrical connectors and wiring harness to the new coil, ensuring proper connections.


The ignition coil plays a vital role in the ignition system of a vehicle. Its function is to generate high voltage, convert low voltage, control timing, and store energy. Understanding the components, working principles, and how to test an ignition coil for faults is crucial for diagnosing and resolving any issues.

By being aware of the symptoms of a faulty ignition coil, using the necessary tools for testing, and interpreting test results accurately, you can identify and troubleshoot problems effectively. Additionally, understanding common issues and their troubleshooting methods can help ensure optimal performance and reliability of the ignition coil.

If a faulty ignition coil is diagnosed, replacing it with a new one following the proper steps will restore the ignition system’s functionality. Regular maintenance and periodic testing of the ignition coil can help prevent unexpected failures and ensure smooth operation of the vehicle’s engine.

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